Among combustion chambers of internal combustion engines which are designed to achieve a high efficiency combustion by directly injecting the fuel into the combustion chamber, there is a method known as the MAN-M method. In the MAN-M method, a main combustion chamber is formed by making a deep spherical recess in the piston top, and while generating a swirl therein, the fuel mist injected from the fuel injection nozzle is allowed to adhere to the inner walls of the main combustion chamber in the form of a liquid fuel film, and the surface evaporation rate of this fuel film is controlled by the swirl.
However, a problem exists with the MAN-M method in that a large amount of HC (hydrocarbons) and bluish white smoke are generated during starting-up at low temperature, such as when the environmental air temperature and the engine cooling water temperature are both low, or when idling while the combustion chamber wall temperature has not yet risen sufficiently. This is because, in the cases mentioned above, the amount of fuel vaporized for the combustion becomes small, and at the same time, the air-fuel mixture (called `mixture` hereinafter) becomes very lean, since as the small quantity of the vaporized fuel is revolved around and carried by the swirl, dispersion of the mixture is assisted in the combustion chamber as a whole.
To solve the above problems, internal combustion engines described in the specifications of Japanese Patent Laid Open No. 85519/81 (called `Proposal No. 1.degree. hereinafter), Japanese Utility Model Laid Open No. 115515/81 (called `Proposal No. 2` hereinafter), Japanese Utility Model Laid Open No. 172125/82 (called `Proposal No. 3` hereinafter), and Japanese Utility Model Laid Open No. 33221/82 (called `Proposal No. 4` hereinafter) have been proposed.
In Proposal No. 1, as shown in FIG. 7, there provided are an ignition chamber (g) in the cylinder head (a), and a main combustion chamber (i) in the piston (h), the two being in communication with each other by a passage (e.sub.1), and a fuel injection nozzle (j) being disposed in an upper part of the ignition chamber (g) at the inner wall (g.sub.1) thereof. From the fuel injection nozzle, fuel is injected toward the inner wall (g.sub.1) of the ignition chamber (g) and the main combustion chamber (i) through the passage (e).
In Proposal No. 2, as shown in FIG. 8, a fuel injection nozzle (k) is provided on the side of the main combustion chamber (i.sub.1) in such a way that most of the fuel injected from the fuel injection nozzle (k) will be sprayed into the subcombustion chamber (m) through the hole (l), on one hand, and to enlarge the cross section area of the injection passage that connects the subcombustion chamber (m) with the main combustion chamber (i.sub.1), on the other hand.
In Proposal No. 3 , as shown in FIG. 9, a fuel injection nozzle (k.sub.1) is disposed in an upper part of the swirl chamber (n), which communicates with the main combustion chamber (i.sub.2) by the passage (e.sub.2), in such a way that the fuel will be supplied not only to the swirl chamber (n) but also to the main combustion chamber (i.sub.2).
In Proposal No. 4, as shown in FIG. 10, the arrangement is to provide a pair of swirl chambers (c.sub.1) and (c.sub.2) in the cylinder head (a) and in the piston top (b), respectively, with their respective air intakes (d) facing each other over a connecting hole (e), in such a way that a part of the fuel mist from the fuel injection nozzle (f) will be injected into the swirl chamber (c.sub.1), in the piston top (b) through the connecting hole (e), with the rest being injected into the swirl chamber (c.sub.2) in the cylinder head (a).
In all of the proposals mentioned above, attempts are made to improve the combustion efficiency by optimizing the air-fuel ratio, i.e., the ratio in which the air that is heated to a high temperature by the compression action of the piston and the injected fuel are mixed. However, to supply the fuel to the main combustion chamber (i, i.sub.2) through the connecting passage (e.sub.1, e.sub.2) as in Proposal No. 1 (FIG. 7) or in Proposal No. 3 (FIG. 9) does mean that the penetration power of the fuel mist, given thereto originally on injection, will be reduced by the flow of air generated in the compression stroke. Therefore, difficulties arise in such process, in that fuel is formed, vaporized, and combusted in the main combustion chamber (i, i.sub.2).
Also, in Proposal Nos. 1 and 3, the chamber that generates and utilizes the swirl is limited to one side only. Therefore, these proposals do not improve the vaporization and the combustion by optimizing the air-fuel reaction (the ratio of air and fuel), and the resulting improvement is insufficient in terms of fuel consumption and output.
In Proposal No. 4, on the other hand, the construction arrangement in which the facing swirl chambers (c.sub.1) and (c.sub.2) which communicate with each other by the connection hole (e) simply doubles the throttling loss of the combustion air forced into the piston top, increases the thermal loss in the flow of the burnt gas into the piston top (b) and leads to deterioration of fuel consumption.
Also, to inject fuel into the swirl chamber (c.sub.1) in the piston top (b) through the connection hole (e) is counterproductive to improvement of diffusivity of fuel mist (a property of the injected fuel mist is to disperse conically with respect to the direction of injection), resulting in reduction in the fuel mist penetration. There is a possibility, furthermore, that the fuel which is injected toward the swirl chamber (c.sub.1) in the piston top (b) is burned in the connecting hole (e) by the combustion energy generated inside the swirl chamber (2) in the cylinder head (a). Therefore, Proposal No. 4 leaves much to be desired with regard to improving fuel consumption and output.